Implemented ratings and other toggle buttons

Implemented 5 toggle buttons that allow the user to show various information either for all images or only the active image. This includes ratings, color labels (traditional and background tint style), history label, and reject cross. In addition, the colors for the rating stars are now read from darktable.gtkrc.in.

see it in action here: http://www.youtube.com/watch?v=Yexe33P8GBM&feature=youtu.be